Place lamb pieces and potato in a large bowl. Add turmeric and salt. Mix well. Cover and rest in the fridge for 30 minutes.Place onion, garlic, ginger, coriander leaves and chilli in the jug of a blender with 1 cup of water. Process until smooth. Set aside.In a large heavy bottomed saucepan or cast iron casserole, heat ghee on high. Add cinnamon, bay leaf, coriander seeds, fennel seeds and black cardamom. Sauté for a few seconds. Add the marinated meat and potatoes. Sauté for a few minutes, tossing occasionally until meat is sealed on the outside and starting to brown. Add the onion mixture and reduce heat to medium.

Add cumin, garam masala and tomatoes. Season with salt to taste. Cover and simmer on medium for 40 minutes. Reduce heat to low. Add coconut milk. Cover and simmer for a further 15 minutes. Serve hot with steamed rice.